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Research Papers: Materials and Fabrication

Brittle Failure Assessment of a PWR-RPV for Operating Conditions and Loss of Coolant Accident

[+] Author and Article Information
Dieter Siegele

Fraunhofer Institute for Mechanics of Materials, Wöhlerstraße 11, 79108 Freiburg, Germanydieter.siegele@iwm.fraunhofer.de

Igor Varfolomeyev

Fraunhofer Institute for Mechanics of Materials, Wöhlerstraße 11, 79108 Freiburg, Germanyigor.varfolomeyev@iwm.fraunhofer.de

Gerhard Nagel

E.ON Nuclear Power GmbH, Tresckowstraße 5, 30457 Hannover, Germanygerhard.nagel@eon-energie.com

J. Pressure Vessel Technol 130(3), 031403 (Jun 06, 2008) (8 pages) doi:10.1115/1.2937759 History: Received September 03, 2005; Revised May 18, 2007; Published June 06, 2008

Abstract

The brittle failure assessment for the reactor pressure vessel (RPV) of a $1300MW$ pressurized water reactor was revised according to the state of the art. The RPV steel is 22 NiMoCr 37 (A 508 Cl. 2). The expected neutron fluence at the end of license (EOL) after $32years$ of full operation is $Φ<2.3×1018neutrons∕cm2$. The assessment followed a multibarrier concept to independently prove the exclusion of crack initiation, crack arrest, and exclusion of the load necessary to advance the arrested cracks through the RPV wall. Thermal and structural analyses of the RPV were performed both for the reactor shutdown with postulated upset conditions, as the most severe load case at operation, and for loss of coolant accident (LOCA) conditions. For LOCA transients, a leak size screening of different combinations of cold∕hot leg injection of emergency core cooling was performed, and the leading leak size was determined. A fracture mechanics based assessment was carried out for extended circumferential flaws in the weld joint between the RPV shell and the flange, as well as for axial flaws in the nozzle corner. These flaw geometries postulated at locations of the highest principal stresses and lowest temperatures under the respective transient conditions are representative for the brittle failure assessment of the whole vessel. For a normal operation, the maximum crack driving force takes place at high temperatures preceding the upset conditions. The transient follows a load path decreasing with temperature, producing a warm prestressing effect, which is considered in the assessment. Thus, a large safety margin against crack initiation can be demonstrated. At LOCA, the most severe conditions are determined for postulated cracks in the nozzle corner. Here, applying the constraint modified master curve, which takes account of the low stress triaxiality in the component, the exclusion of crack initiation is proven. Furthermore, two additional safety barriers are proven, the crack arrest after postulated crack initiation well within the allowable depth, as well as the preclusion of the load necessary to advance the arrested crack through the RPV wall.

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Figures

Figure 1

Multibarrier safety concept for RPV assessment

Figure 2

Stress-strain curves of the base metal at 20°C and 300°C

Figure 3

Water temperature and internal pressure at shutdown followed by an upset condition at 50°C

Figure 4

Fluid temperatures in the cold leg nozzle for a postulated 10cm2 hot leg leak

Figure 5

FE models: (a) 22.5deg section of RPV, (b) circumferential crack in the flange joint, and (c) axial crack in the nozzle corner

Figure 6

Semielliptical circumferential surface crack in the flange joint under a normal operation ending at 50°C and followed by an upset condition: load path at the center of the crack front

Figure 7

Axial nozzle corner crack under a normal operation ending at 50°C and followed by an upset condition: load path at the center of the crack front

Figure 8

Quantification of the WPS effect for different load paths. Example: LPUCF path (8)

Figure 9

Compilation of load path maxima for all crack depths and leak transients investigated for the flange joint

Figure 10

Compilation of load path maxima for all crack depths and leak transients investigated for the nozzle corner

Figure 11

Load path maxima for circumferential cracks of varying crack depths in the flange joint for the most severe leak transients

Figure 12

(a) Maximum principle stress and (b) stress triaxiality ahead of the crack front in the nozzle corner and in a C(T) specimen

Figure 13

T-stress estimates for a 20mm deep crack in the nozzle corner

Figure 14

Load path maxima for axial cracks of varying depths in the nozzle corner for the 10cm2 leak transient compared with the constraint representative fracture toughness curve

Figure 15

Crack arrest diagram for nozzle corner cracks for the 10cm2 leak transient

Figure 16

J-integral in the nozzle corner for the arrested crack versus internal pressure

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